The disclosure relates generally to turbine components, and more particularly, to a turbine component including a tip rail cooling passage.
In a gas turbine engine, it is well known that air is pressurized in a compressor and used to combust a fuel in a combustor to generate a flow of hot combustion gases, whereupon such gases flow downstream through one or more turbines so that energy can be extracted therefrom. In accordance with such a turbine, generally, rows of circumferentially spaced turbine rotor blades extend radially outwardly from a supporting rotor disk. Each blade typically includes a dovetail that permits assembly and disassembly of the blade in a corresponding dovetail slot in the rotor disk, as well as an airfoil that extends radially outwardly from the dovetail.
The airfoil has a generally concave pressure side wall and generally convex suction side wall extending axially between corresponding leading and trailing edges and radially between a root and a tip. It will be understood that the blade tip is spaced closely to a radially outer turbine shroud for minimizing leakage therebetween of the combustion gases flowing downstream between the turbine blades. Maximum efficiency of the engine is obtained by minimizing the tip clearance or gap such that leakage is prevented, but this strategy is limited somewhat by the different thermal and mechanical expansion and contraction rates between the turbine rotor blades and the turbine shroud and the motivation to avoid an undesirable scenario of having excessive tip rub against the shroud during operation.
In addition, because turbine rotor blades are bathed in hot combustion gases, effective cooling is required for ensuring a useful part life. Typically, the blade airfoils are hollow and disposed in fluid communication with the compressor so that a portion of pressurized air bled therefrom is received for use in cooling the airfoils, as a coolant. Airfoil cooling is quite sophisticated and may be employed using various forms of internal cooling channels and features, as well as cooling holes through the outer walls of the airfoil for discharging the cooling air. Nevertheless, airfoil tips are particularly difficult to cool since they are located directly adjacent to the turbine shroud and are heated by the hot combustion gases that flow through the tip gap. Accordingly, a portion of the air channeled inside the airfoil of the blade is typically discharged through the tip for the cooling thereof.
It will be appreciated that conventional blade tips include several different geometries and configurations that are meant to prevent leakage and increase cooling effectiveness. Conventional blade tips, however, all have certain shortcomings, including a general failure to adequately reduce leakage and/or allow for efficient tip cooling that minimizes the use of efficiency-robbing compressor bypass air. One approach, referred to as a “squealer tip” arrangement, provides a radially extending rail that may rub against the tip shroud. The rail reduces leakage and therefore increases the efficiency of turbine engines.
However, the rail of the squealer tip is subjected to a high heat load and is difficult to effectively cool—it is frequently one of the hottest regions in the blade. Tip rail impingement cooling delivers coolant through the top of the rail, and has been demonstrated to be an effective method of rail cooling. However, there are numerous challenges associated with exhausting a coolant through the top of the rail. For example, backflow pressure margin requirements are difficult to satisfy with this arrangement (especially on the pressure side wall, where there are holes connected to low and high pressure regions—the top and pressure side walls of the rail, respectively). Hence, it is a challenge to create losses in the tip passage to back-pressure the coolant flow, and at the same time, sufficiently cool the rail, since losses reduce the amount of cooling fluid used in this region. Further, the outlet holes must exhibit rub tolerance yet provide sufficient cooling to the rails. For example, the outlet holes must be tolerant of tip rub but also sufficiently large that dust cannot clog them.
Ideally, the rail cooling passages are also capable of formation using additive manufacturing, which presents further challenges. Additive manufacturing (AM) includes a wide variety of processes of producing a component through the successive layering of material rather than the removal of material. As such, additive manufacturing can create complex geometries without the use of any sort of tools, molds or fixtures, and with little or no waste material. Instead of machining components from solid billets of material, much of which is cut away and discarded, the only material used in additive manufacturing is what is required to shape the component. With regard to tip rail cooling passages, conventional circular cooling holes within the rail are very difficult to build using additive manufacturing (perpendicular to the nominal build direction) and severely deform or collapse during manufacture.
Another challenge with tip cooling is accommodating the different temperatures observed in different areas of the tip rail. For example, the rail in the pressure side wall and aft region of the suction side wall are typically hotter than other areas.